United Nations secretary general António Guterres has appealed to Russia, Iran and Turkey to “spare no effort to find solutions that protect civilians” in Idlib and said it was “absolutely essential” to avoid a full-scale battle in Syria’s rebel-held province.

“This would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict,” he told reporters.

Civilians in Syria's last rebel stronghold brace for final battle

Syrian government and Russian warplanes began airstrikes in Idlib last week in a possible prelude to a full-scale offensive to retake the region. They say they are targeting terrorist groups.

“I understand that the present situation in Idlib is not sustainable and the presence of terrorist groups cannot be tolerated. But fighting terrorism does not absolve warring parties of their core obligations under international law,” Guterres told reporters in New York.

His comments came after Turkey renewed calls for international backing for a ceasefire, telling the UN security council that an all-out assault would trigger a huge wave of refugees and could threaten Europe.

Russia called for the council meeting to brief members on a summit it held with Iran and Turkey on military plans to retake Idlib, the last major rebel stronghold in Syria.

“There is no doubt that an all-out military operation would result in a major humanitarian catastrophe,” the Turkish ambassador, Feridun Sinirlioğlu, told the council meeting.

Further airstrikes and bombings would trigger a “massive wave of refugees and tremendous security risks for Turkey, the rest of Europe and beyond,” he warned.

Turkey, which has sent troops to Idlib and supports some of the armed groups, called for an immediate ceasefire and urged the international community “to vocally and actively support our efforts to this end”.

The US ambassador, Nikki Haley, said Syrian forces, backed by Russia and Iran, had already launched 100 airstrikes on Idlib this month and that their sole aim was “a bloody military conquest of Idlib”.

Haley warned of dire consequences if mass casualties were caused if the assault went ahead. “The world will hold them responsible,” she added.

Britain and France backed the call for a ceasefire, recalling that these had been agreed under arrangements for a “de-escalation zone” in Idlib that were overseen by Russia, Iran and Turkey.

Russia and Iran insisted that a military offensive in Idlib would be a counter-terrorism operation and that measures would be taken to spare civilians.

“We cannot allow terrorists to hold hostage and use hundreds of thousands of people of Idlib as human shields,” said the Russian ambassador, Vassily Nebenzia.

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However Russia’s Syria envoy said on Tuesday it would be up to Turkey to separate terrorist elements from the moderate opposition.

“We are saying the situation in Idlib should be settled most preferably in a peaceful way. It is possible to abstain from using military force,” Alexander Lavrentiev told reporters following talks in Geneva with the UN envoy Staffan de Mistura.

“Idlib province is … a sort of zone of responsibility of Turkey; it is their responsibility to separate the moderate opposition from the extremists, from Jabhat al-Nusra and other groups, other terrorist groups,” he said.

The UN security council meeting came days after it heard warnings from De Mistura about a looming catastrophe from the military offensive in Idlib.